Eclipses have long played a part in understanding our atmosphere, as an
eclipse is the nearest nature comes to turning off the sun. When this
happens, the results in the upper atmosphere can be dramatic.
The temperature drops, changing the wind pattern as air contracts in on
the eclipse region. In the absence of the ionising radiation from the sun,
our ionosphere rapidly decays, and this will affect the propagation of radio
signals.
The shadow of the moon races through the atmosphere at supersonic speeds,
causing waves to spread through the atmosphere from the eclipse region.

All of these phenomena have been predicted by theory, and some have been
observed. A collaboration of scientists from universities and government
research laboratories intend to use the 1999 eclipse as an opportunity to
make a comprehensive set of ionospheric measurements.
For more information, follow one of the hotlinks below.